I. Technical Field
This invention relates to linear frequency modulation (FM) sweep generator systems. More specifically, it relates to a linear FM sweep generator having a novel adaptive linearity correction system.
II. Background Art
Linear FM sweep generators produce an output signal in which the frequency of the output signal changes linearily as a function of time. In many applications of the linear FM sweep generator, it is necessary for the linearily swept output signal to be as precise and accurate as possible. However, due to nonlinearities inherent in the generator system and external factors such as temperature, humidity or age, the linearly-swept output signal may become nonlinear. The frequency nonlinearities in the output signal adversely affect the performance of the apparatus in which the linear FM sweep generator is used.
Linear FM sweep generators are generally utilized in test equipment applications where a linear FM sweep output is required, such as in a spectrum analyzer. Another application of the linear FM sweep generator is in radar systems. In particular, high-resolution radar systems may employ linear FM to achieve high levels of pulse compression. To realize the theoretical range resolution of these radar systems, especially with long correlation delays between the reference sweep and the received echos, a high degree of frequency sweep linearity is required. In addition, frequency sweep linearity is necessary to achieve low compressed pulse time sidelobe levels. The requirement of frequency sweep linearity applies both to high-resolution radar systems that (1) simultaneously transmit a reference sweep waveform while receiving an echo, and those which (2) reconstruct a reference sweep waveform beginning at the time of reception of echos from the range window of interest.
Open-loop pre-compensation is one technique used for compensating nonlinearities of a linear FM sweep generator having a voltage-controlled oscillator (VCO) driven by a control voltage waveform generator. This technique requires one to passively tailor the control voltage waveform to compensate for nonlinearities inherent in the VCO tuning characteristics, thereby improving the frequency sweep linearity of the output signal by reducing sweep-to-sweep correlated linearity errors.
To further improve a linear FM sweep generator, a closed-loop servo may be added. The closed-loop servo generally includes a frequency or phase detector, a sampling frequency generator, and a correction circuit so as to provide a servo error signal to the VCO. The closed-loop servo detects instantaneous frequency or phase deviations in the linear frequency sweep with respect to an ideal linear frequency sweep. The output signal of the VCO is sampled by the phase or frequency detector at a rate determined by the sampling frequency generator. The detected errors are used in a correction circuit to generate corresponding servo error signals. The servo error signals are applied to the input of the VCO along with the output of the control voltage waveform generator. The servo error signal provides the required linearity correction in the output signal.
Further improvements in the effectiveness of a closed-loop servo may be realized when it is combined with the open-loop pre-compensation technique. However, the open-loop pre-compensation in combination with a general closed-loop servo still suffers from sweep nonlinearities. With the open-loop pre-compensation technique, nonlinearity compensation is only as good as the initial matching of the control voltage waveform to the defined nonlinearity characteristics of the VCO. The defined nonlinearity characteristics of the VCO may change with age, temperature and/or various other causes thereby rendering the precompensation ineffective.
Generally, the closed-loop servo is designed to correct rapidly varying sweep linearity errors within the sweep. In addition to correcting the instantaneous sweep linearity errors within the sweep, the closed-loop servo during each sweep must also correct sweep linearity errors that are correlated from sweep-to-sweep. Correcting both correlated and uncorrelated errors places an onerous burden upon the closed-loop servo. The correlated errors may eventually exceed the dynamic range characteristics of the closed-loop servo resulting in a limitation of effective error correction.
It is therefore an object of the present invention to provide an adaptive linear FM sweep correction system that improves the sweep linearity of closed-loop linear FM sweep generators.
It is another object of the present invention to provide an adaptive closed-loop FM sweep generator by extracting additional information from the closed-loop servo error signal and utilizing the additional information to eliminate sweep-to-sweep correlated errors, thus improving the open-loop tuning accuracy of the VCO and the overall closed-loop sweep linearity.
It is yet another object of the present invention is to provide an improved highly-accurate linear FM sweep generator which automatically compensates for variations in the VCO frequency vs. voltage characteristics as a result of component mismatches; aging; fluctuations in power supply output voltages; or changes in environmental factors such as temperature, barometric pressure.
It is still another purpose of the present invention to provide an improved precision linear FM sweep generator which automatically compensates for sweep nonlinearities caused by frequency-dependent effects of components in the signal path between the VCO output and the sweep error measurement point, including effects due to inherent component defects, antenna reflections or component mismatches.
It is further purpose of the present invention to provide an adaptive linear FM sweep generator which generates accurate linear FM swept waveforms in a relatively simple and inexpensive manner by digital implementation of an active open-loop VCO tuning controller.